Introduction to Biology

• Biology is the scientific study of life
• It explores the structure, function, growth, origin, evolution, distribution, and taxonomy of living organisms

Core Principles of Biology

• Cell theory: The cell is the basic unit of life
• All living organisms are composed of one or more cells
• All cells arise from pre-existing cells
• Gene theory: Genes are the basic units of heredity
• Genes are composed of DNA and encode proteins
• Genes are passed from parents to offspring
• Evolution: Life evolves through natural selection
• Organisms change over time in response to their environment
• Evolution explains the unity and diversity of life
• Homeostasis: Living organisms maintain a stable internal environment
• Organisms regulate their internal conditions such as temperature, pH, and water balance

Levels of Biological Organization

• Atoms: The basic units of matter
• Molecules: Two or more atoms bonded together
• Organelles: Structures within cells that perform specific functions
• Cells: The basic unit of life
• Tissues: Groups of similar cells performing a specific function
• Organs: Structures composed of different tissues working together
• Organ systems: Groups of organs working together
• Organisms: Individual living beings
• Populations: Groups of individuals of the same species living in the same area
• Communities: Populations of different species living in the same area
• Ecosystems: Communities of organisms interacting with their physical environment
• Biosphere: All of the ecosystems on Earth

Branches of Biology

• Biochemistry: The study of the chemical processes within living organisms
• Botany: The study of plants
• Cell biology: The study of cells
• Ecology: The study of the interactions between organisms and their environment
• Evolutionary biology: The study of the evolutionary processes that have shaped life on Earth
• Genetics: The study of heredity
• Microbiology: The study of microorganisms
• Molecular biology: The study of the molecular basis of life
• Zoology: The study of animals

Cell Biology

• Cell biology is the study of cells, their structure, function, and behavior
• It encompasses the physiological properties of cells, their interactions with the environment, their life cycle, division, and death
• Cell biology research includes both prokaryotic and eukaryotic cells

Cell Structure

• Cells are typically microscopic and consist of several key components
• Plasma membrane: outer boundary of the cell, controls what enters and exits
• Cytoplasm: the region between the plasma membrane and the nucleus, contains organelles
• Nucleus: contains the cell's genetic material (DNA)
• Organelles: membrane-bound structures within the cytoplasm that perform specific functions

Prokaryotic Cells

• Lack a nucleus and other membrane-bound organelles
• Genetic material (DNA) is located in the cytoplasm in a region called the nucleoid
• Generally smaller and simpler than eukaryotic cells
• Include Bacteria and Archaea

Eukaryotic Cells

• Have a nucleus and other membrane-bound organelles (mitochondria, endoplasmic reticulum, Golgi apparatus)
• More complex and generally larger than prokaryotic cells
• Found in plants, animals, fungi, and protists

Key Organelles and Their Functions

• Nucleus: Contains the cell's DNA and controls cell activities
• Endoplasmic reticulum (ER): involved in protein and lipid synthesis
• Rough ER: contains ribosomes for protein synthesis
• Smooth ER: involved in lipid synthesis and detoxification
• Golgi apparatus: processes and packages proteins and lipids
• Mitochondria: produce energy (ATP) through cellular respiration
• Lysosomes: contain enzymes for intracellular digestion
• Peroxisomes: involved in detoxification and metabolism of fatty acids
• Ribosomes: synthesize proteins
• Cytoskeleton: provides structural support and facilitates cell movement
• Microfilaments (actin filaments): involved in cell movement and muscle contraction
• Intermediate filaments: provide structural support
• Microtubules: involved in cell division and intracellular transport
• Cell wall (in plant cells): provides support and protection

Plasma Membrane

• A selectively permeable barrier that controls the movement of substances into and out of the cell
• Composed of a lipid bilayer with embedded proteins
• Lipids: phospholipids, cholesterol
• Proteins: transport proteins, receptors, enzymes

Transport Across the Plasma Membrane

• Passive transport: does not require energy
• Diffusion: movement of molecules from an area of high concentration to an area of low concentration
• Osmosis: movement of water across a semipermeable membrane from an area of high water concentration to an area of low water concentration
• Facilitated diffusion: diffusion of molecules across a membrane with the help of transport proteins
• Active transport: requires energy (ATP)
• Movement of molecules against their concentration gradient, with the help of transport proteins
• Bulk transport: movement of large molecules or particles across the plasma membrane
• Endocytosis: the cell takes in substances by engulfing them in a vesicle
  • Phagocytosis: "cell eating" - engulfing large particles or cells
  • Pinocytosis: "cell drinking" - engulfing fluids and small solutes
  • Receptor-mediated endocytosis: binding of specific molecules to receptors on the cell surface triggers vesicle formation
• Exocytosis: the cell releases substances by fusing a vesicle with the plasma membrane

Cell Communication

• Cells communicate with each other through various signaling mechanisms
• Direct contact: communication through cell junctions
• Local signaling: communication between nearby cells
• Paracrine signaling: signaling molecules released by one cell affect nearby cells
• Synaptic signaling: communication between nerve cells
• Long-distance signaling: communication between cells that are far apart
• Endocrine signaling: hormones released by endocrine cells travel through the bloodstream to target cells

Signal Transduction Pathways

• A series of steps by which a signal on the cell surface is converted into a specific cellular response
• Involves receptor proteins, intracellular signaling molecules, and effector proteins

Cell Cycle

• The series of events that take place in a cell leading to its division and duplication of its DNA (replication) to produce two new daughter cells
• Consists of two major phases: interphase and mitotic phase (M phase)
• Interphase: cell grows and prepares for division
• G1 phase: cell growth and normal functions
• S phase: DNA replication
• G2 phase: preparation for cell division
• Mitotic phase (M phase): cell divides
• Mitosis: division of the nucleus
  • Prophase, prometaphase, metaphase, anaphase, telophase
• Cytokinesis: division of the cytoplasm

Cell Cycle Control

• Regulated by internal and external signals
• Checkpoints: control points in the cell cycle where progress is halted until certain conditions are met
• G1 checkpoint: checks for DNA damage and cell size
• G2 checkpoint: checks for DNA replication and cell size
• M checkpoint: checks for chromosome alignment

Cell Death

• Apoptosis: programmed cell death; is a normal part of development and tissue maintenance
• Necrosis: cell death due to injury or disease; often causes inflammation

Cell Metabolism

• Metabolism: all chemical reactions that occur within a cell
• Catabolism: breakdown of complex molecules into simpler ones, releasing energy
• Anabolism: synthesis of complex molecules from simpler ones, requiring energy
• Enzymes: proteins that catalyze (speed up) chemical reactions

Cellular Respiration

• The process by which cells break down glucose to produce energy (ATP)
• Consists of several stages:
• Glycolysis: breakdown of glucose into pyruvate
• Pyruvate oxidation: pyruvate is converted to acetyl CoA
• Citric acid cycle (Krebs cycle): acetyl CoA is oxidized, releasing carbon dioxide and generating ATP, NADH, and FADH2
• Oxidative phosphorylation: electrons from NADH and FADH2 are used to generate a proton gradient across the mitochondrial membrane, which drives ATP synthesis

Photosynthesis

• The process by which plants and other organisms use light energy to convert carbon dioxide and water into glucose
• Occurs in chloroplasts
• Consists of two main stages:
• Light-dependent reactions: light energy is captured and converted into chemical energy (ATP and NADPH)
• Light-independent reactions (Calvin cycle): ATP and NADPH are used to convert carbon dioxide into glucose